1. Field of the Invention
The present invention relates to exposure control devices for photographic cameras and in particular to such devices wherein the output signal of a light measuring circuit is smoothed and used for exposure control while the light measuring circuit is measuring an object or scene light with its intensity including a component that changes with high frequency.
2. Prior Art
A diaphragm control device has been proposed wherein an electronic flash tube is actuated prior to a film exposure to illuminate an object to be photographed with a light of a constant intensity. The light from the object is measured by a light measuring circuit through the fully open diaphragm, known photographic calculation is made with the output signal from the light measuring circuit and a signal as a function of the F number of the photographic lens. The result of the calculation is stored in a storage circuit, the diaphragm is stopped down with the object light being measured through the diaphragm aperture (the pre-exposure illumination by the electronic flash tube being continued also in this time), and the output of the light measuring circuit is compared with the value stored in the aforementioned storage circuit to stop the operation of the diaphragm and determine the aperture size when both values being compared attain a proportionate relationship.
In a device such as the above, the pre-exposure flash light of a constant intensity is obtained by intermittently providing high periodic current to the flash discharge tube. Therefore, it actually embodies a high frequency component in the illuminated light even though in effect it has a constant intensity. It is therefore possible that a highly responsive photoelectric element is liable to respond to the change of the light intensity, resulting in a control error. The present invention provides a smoothing circuit between the light measuring circuit and the exposure control circuit for the purpose of eliminating such defects.
A known circuit structure includes a ripple-smoothing resistance between the output of an photoelectric circuit and a control circuit so that, when an object brightness is measured with the object being illuminated by a light source energized by an AC commercial power source, the output will correspond to an average value of the AC component of the brightness.
When the absolute value of an object brightness is necessary such as in the case of shutter speed control according to an measurement of an object's brightness, the average value of the AC component in the brightness must be detected. However, in the automatic diaphragm determination for flash photography mentioned above, the absolute value of the light intensity should not necessarily be determined so long as the object is illuminated with constant light from the time when the diaphragm is fully opened until the stopping-down operation of the diaphragm is stopped. Additionally as the pre-exposure flash light emission must be maintained for a given period with the flash tube being energized by a capacitor which is charged by a voltage boosted from an output of a battery of a small capacity, the intensity of the flash light can not be high. Therefore, the circuit for diaphragm control should preferably utilize the light reflected from the object most effectively.